really sick? In the emergency room, real, life-threatening illness is often spotted as quickly as pornography and poor fashion sense — we know it when we see it. But for most people, in most situations, sorting out true illness can be difficult.

Many healthy folks cycle through ERs for "nothingitis" because they are anxious about disease, whilst some very unwell people stick it out at home, in full-on denial or stoically convinced that they can will themselves better.

Wouldn't it be nice to have a reliable sense of sickness — an epiphany of medical impairment? Perhaps some of us already do. Take, for example, the "aura," and by this I'm not referring to the luminous projection of light hovering over Mother Teresa, but to a well-described medical phenomenon.

"The process which precedes the headache of migraine is very mysterious ... There is a process of intense activity which seems to spread, like the ripples in a pond into which a stone is thrown ... The most frequent among the many forms is that of a small star near the fixing point; it enlarges towards one side, its rays expanding into zigzags."

So wrote British neurologist Sir William Gowers in 1906 in the British Medical Journal. Gowers was fascinated with the "aura" — a premonition of a medical process to come. Auras, also common in people with seizure disorders, come in a wide variety of forms — a kaleidoscope of lights, the smell of burnt toast, the sound of a public address announcer booming — and may occur seconds to hours before the onset of a headache or seizure. For a patient with epilepsy or a migraineur, the aura is an extremely reliable indicator of impending symptoms (far more accurate than routinely available clinical evaluation or testing). It's not well understood where auras come from, but they are thought to be associated with excitation of specific areas of the brain preceding the onset of more generalized processes.

Is it possible that auras are prominent manifestations of an innate mental ability to detect illness — a sense and premonition of sickness?

Consider some other health-related premonitions.

It is not uncommon for pregnant women who miscarry or otherwise lose their pregnancy to experience a premonition beforehand. For example, in a survey study of women who suffered a stillbirth in the second trimester or later, 64 percent reported some premonition that their child was unwell.

Case reports of patients' having a "premonition of death" are sprinkled through the literature. Joseph Ngeh, in a 2003 letter to the Journal of the American Geriatrics Society, describes one such premonition in the in-hospital death of an elderly patient: The patient's family had arrived by then. Although distraught, they showed no surprise at hearing about the patient's sudden death. During our conversation, I sensed that they had expected this to happen. Remarkably, the daughter-in-law volunteered that, when they visited the patient at 9 p.m., earlier that night, a mere 6 hours before the patient's first cardiopulmonary arrest the patient had held her hand and mentioned that he would 'die tonight.'"

Premonitions of death also are common in trauma patients. In a 2009 survey-based study by Miglietta and colleagues of 302 members of the Eastern Association for the Surgery of Trauma, 95 percent of respondents reported encountering patients who expressed premonitions of death and 50 percent agreed that patients expressing such premonitions had a higher mortality rate.

Now, such evidence must be considered in the light of chance occurrence — there are surely many pregnant women and trauma patients who thrive or recover despite premonitions to the contrary. And, absent a biologic explanation, it's impossible to prove that humans have an innate sense of sickness.

Wouldn't it be nice, though, to intuitively predict the onset of a heart attack or a stroke? And do it as confidently as an aura predicts a seizure? This is not likely to be possible anytime soon, but there are some tips and tools that we can all use to get in better touch with our own sense of sickness (and health).

It turns out that some symptoms, or groups of symptoms, are far more concerning that others. For example, sharp chest pain, lasting only seconds at a time is much less worrisome than a deep heaviness that is associated with difficulty breathing. Some other time we will dive deeper into such tips and tools and some of the evidence supporting them.

Dr. Dustin W. Ballard is an emergency physician at Kaiser Permanente San Rafael and the author of "The Bullet's Yaw: Reflections on Violence, Healing and an Unforgettable Stranger." His Medically Clear column appears every third Monday; follow him on Twitter at http://twitter.com/dballard30.